The present invention is related to a pipetting device with means for detachably connecting with at least one pipette tip, at least one displacement device, a driving device for driving a displacement member of the displacement device and an adjustment device with a lead screw nut and a lead screw for adjusting the metering volume.
Pipetting devices are used in the laboratory, in particular, for the metering of liquids. The liquids are picked up and discharged in pipette tips. In air cushion pipettes, a displacement device for a gas is integrated into the pipetting device and communicatingly connected with at least one pipette tip. An air cushion is displaced by means of the displacement device, so that liquid is sucked into the pipette tip and ejected out of it. Frequently, the displacement device is a cylinder with a plunger movable therein. The means for detachably connecting with at least one pipette tip are realised as a put-up cone or as a blind hole, for instance. A hole in the front side of the put-up cone or in the bottom of the blind hole is connected to a displacement chamber of the displacement device.
The pipette tips are detachably connected to the pipetting device, so that they can be replaced by new pipette tips after use. Through this, carryovers and contaminations can be avoided at subsequent meterings. Pipette tips for one time use are available from plastics at a reasonable price.
The metering volume of the liquid picked up and discharged by means of the pipetting device is adjustable by means of the adjustment device. In order to adjust the metering volume, the stroke travel which the plunger can perform in the cylinder is changed. For this purpose, a stop is shifted which co-operates with a counter-stop on a lifting rod which shifts the plunger, in order to limit the stroke travel of the lifting rod. The stop is arranged on the front side of a lead screw, which is screwable in a lead screw nut stationary with respect to the displacement chamber and which is guided through the lifting rod. The counter-stop is a washer or bead, respectively, projecting radially from the lifting rod. On the other end, the stroke travel is limited by another stop which co-operates with the bead on the lifting rod or the plunger.
In conventional pipetting devices, lead screw nut and lead screw are produced separately and provided with threads. Due to manufacturing tolerances, the lead screw has a clearance in the nut, which may lead to metering inaccuracies.
Further, there are pipetting devices in which the lead screw nut made of plastic material is produced with a threadless bore at first, and the internal thread of the lead screw nut is then formed by screwing in a lead screw made of metal and provided with an external thread into the bore. These realisations avoid a clearance between lead screw nut and lead screw. However, they have the disadvantage that the adjustment of the metering volumes in pipetting devices of different magnitude, which differ in the adjustment ranges of their metering volumes (for instance, 0.5-10 μl, 10-100 μl, 100-1000 μl), is accompanied by a very high expense of force. Namely, due to the thread friction decreasing with increasing thread steepness, the expense of force for adjusting the metering volume is greater at pipetting devices with small adjustment range than the expense of force for adjusting the metering volume at pipetting devices with great adjustment range.
Starting from this, the present invention is based on the objective to provide a pipetting device which can be more easily operated with a defined adjustment force for different adjustment ranges.